1. Field of the Invention
The present invention relates to a call acceptance controlling apparatus and a method for controlling acceptance of new calls and handover calls generated in a cell where communication is provided by a radio channel between a mobile station in the cell and two or more radio base stations that form cells in a mobile communications system that employs a code division multiple access method.
2. Description of the Related Art
Generally a mobile communications system accepts a request for a call to be established by assigning a radio channel. However, in any mobile communications system, the number of available radio channels is limited and, thereby, the number of mobile stations (users) that can communicate simultaneously, i.e., subscriber capacity, is limited. For this reason, when a request for a call exceeding the subscriber capacity is generated, a radio channel cannot be assigned to the requested call, resulting in a so-called lost call.
In a mobile communications system that employs a frequency division multiple access method (FDMA method) and a time division multiple access (TDMA method), for example, fixed radio channels are assigned to each cell formed by a radio base station.
In this case, the number of mobile stations that can communicate simultaneously within a cell is limited by the number of the radio channels assigned to the cell. For this reason, when a request for calls in excess of the number of the radio channels is generated, a call loss occurs.
In a mobile communications system such as above, fixed radio channels are assigned to each cell, making it impossible to flexibly respond to an uneven traffic distribution, location-wise and time-wise. In order to solve the uneven traffic distribution, a method (dynamic channel allocation method) that assigns radio channels dynamically to each cell is sometimes used. In the mobile communications system using this dynamic channel allocation method, a radio channel that satisfies predetermined communication quality (for example, a radio channel with an amount of interference below a predetermined value and a radio channel with CIR being greater than a predetermined value) is assigned. However, when there is no radio channel available, or predetermined communication quality cannot be provided even if a radio channel is available, the radio channel cannot be assigned to the call. Thus, the call is lost.
On the other hand, in a mobile communications system using the code division multiple access (CDMA method), each mobile station shares the same radio frequency band, using a spread code. Under the CDMA method, the radio channel is constituted by the spread code and resistance against interference is strong compared with the narrow-band multiple access method of the FDMA method and the TDMA method. However, when an amount of interference exceeds a certain value, the CDMA has a property that the communication quality deteriorates. For a mobile station using a predetermined spread code, a signal of another mobile station using another spread codes can cause interference.
That is, the subscriber capacity in the mobile communications system using the CDMA method is limited by the amount of interference. For this reason, if a call that will cause the mobile communications system to exceed the subscriber capacity in a certain cell is accepted, communication quality of all mobile stations in the cell and its surrounding cells deteriorates. Here, the amount of interference caused by a communication using a certain spread code with another communication using another spread code is determined by a cross-correlation between these spread codes. For this reason, spread codes are designed so that cross-correlation values becomes sufficiently small.
As regards calls originated in each cell, the calls are classified as new calls that occur by a call origination and handover calls that occur by handover. In the mobile communications system, when a mobile terminal under communication moves from a first cell to a second cell, a radio channel has to be switched from a channel of the first cell to a channel of the second cell. At this time, the call in the first cell turns into a handover call of the second cell. When there is no radio channel assignable to the handover call, the call of the mobile terminal will be compulsorily disconnected in spite of the call being in progress. This forced release has adverse influence on the quality of service.
In order to lessen occurrence of forced disconnections, preparing radio channels exclusively for handover calls is practiced in the narrow-band mobile communications system using the FDMA method or the TDMA method. In the method presented by JP, 7-23449, a plurality of radio channels are divided into a small number of channels that are assigned exclusively to handover calls, a comparatively small number of channels that are assigned to handover calls preferentially, and the rest of a greater number of channels that are assigned to new calls and handover calls without preference. To the new calls, radio channels are assigned according to operating conditions of the radio channels. In this manner, the handover calls can be accepted preferentially and the quality of service can be enhanced.
However, in a mobile communications system using the CDMA method, the subscriber capacity is dependent on an amount of interference rather than available spread code resources. For this reason, the call acceptance method of the narrow-band mobile communications system using the FDMA method or the TDMA method proposed by JP, 7-23449 mentioned above cannot be applied to the mobile communications system using the CDMA method.